i did use Demo Account with FXsolutions for this purpose only, i do not use them for live trades.
you can start a demo with them for this data and when you place a demo trade a box pops up for order entry and allows you to set how much to invest, in your case $ 5 a pip and it will show you what amount of Margin it will take for you to take that size position.
back when i did use their Demo for this you could select up to 400:1
leverage to see what Margin it would take, now that U.S. Brokers are at 50:1 i am not sure you could see accurate info if you wanted to use more leverage than the 50:1, but you just double that amount it is saying for the 50:1 if you wanted to use 100:1.
in other words if your $5/50,000 a pip position at 50:1 is saying it takes $ 1,500 in Margin to take that size position then it will take
$ 3000 if you use 100:1 for that same size position.
your right it does take into account Leverage used and also at what price the GBP is currently at to give you your needed Margin requirement.
The other thing to take into account is your Total Account size, if you were to take that $ 5 a pip position, you still need money above that to be able to stay in the trade, at least hundreds of dollars more, depending on that particular brokers Unused Margin Policy.
I would suggest if taking only one position to have at least double the Margin it Requires to take that Position, so to take into Account for Drawdown and the Brokers Unused Margin Policy.
by the way i am not recommending what leverage to use as Most anyone will tell you Not to trade with High Leverage, to some 50:1 is to High so that will be for you to decide.
Hope that might help you

i need to Correct the above as i made a mistake, the Position size Example at $ 5 a pip with 50:1 leverage would be $ 1,500 and if using 100:1 Leverage it would only take half that amount in Margin
to take that same size Trade= $ 750.00,
Sorry for any Confusion....i got it backwards at first.
anyway Fxsolution Demo will Clearly tell you the Required Margin Needed to take whatever Size Position you choose.